Sound waves which lie below the limit which is perceptible by the human ear of approximately 16 Hz but above natural barometric variations are called infrasound. The lower the frequency is, the more unhindered the infrasound waves propagate. Infrasound which is transmitted through the air can be detected even in a distance of several kilometers from a source; in water, the distance can even be a multiple thereof.
Unnatural resources of infrasound are for example supersonic airplanes, explosions (e.g. also due to tests of weapons), but also industrial plants which use machines, as well as air handling systems. Also wind generators whose moving rotor blades induce a compression of the air between themselves and the tower during passage of the lowest position produce rhythmical low-frequency vibrations. Further, heavy goods vehicle traffic on bridges can generate infrasound. Further sources are e.g. movements of boats or rough sea.
An interest exists for simple, effective measurement methods for infrasound, since it was repeatedly reported that infrasound would have negative effects on well-being and health. But also the possibility of damages of buildings because of permanent infrasound exposure requires its determination.
However, measurement of such sound waves is difficult, because in particular in nature, manifold interferences occur which overlap the infrasound and impede the identification of a source. This can e.g. be wind, air handling systems, trucks, trains, flowing waters, or rain.
While common acoustic sound pressure microphones do extend, if at all, then not far into the infrasound region, pressure sensors are usually not sensitive enough. Further, measurement by means of special devices such as micro barometers is expensive.